Greetings.
I have been working with 2 plasma activities for a while with 2 screen monitors. As expected each screen shows each activity and the swap of activities through the screens always worked like a charm (activity tabs plasmoid).

Today I added a new plasma activity and the plasmoid shows it up. I didn't even thought what may happen with if I don't have another screen for it and so just after a few swaps plasma activity tabs stopped working.
I mean I can't change my activities any more (have to reload x). The plasmoid doesn't answer my clicks and so I can't access to a third activity.

Is this a bug or a KDE Plasma rule that you can not work with more activities than screens? Actually it looks like you have to work with a non odd relation between activities and screens isn't it? Sounds like I should plug another monitor for my third activity

My experience with activities with two monitors (screens) is that each screen switches activities independently. I used to use the activity switcher plasmoid, but in KDE 4.4, they added the ability to bind a mouse actions to various other actions like show menu, and switch activity (on a per-activity basis). So I set scroll to switch activity on each of my activities (I have 5 right now) and use that (I also have a keyboard shortcut to switch activity as well, which I think you could configure those as of KDE 4.3).
As I recall, when I would use the activity switcher plasmoid to switch one screen to the activity that the other screen is on, plasma would freeze (though that may have been as much an artifact of my using KDE on Windows XP; I'm not sure if that problem is generic across platforms).

You might try setting up a keyboard shortcut to switch activity (and if you have KDE 4.4, set up the mouse actions) and see if it works there.

Your experience of switching activities independently on each screen looks like what I need to find.
I forgot the mouse actions and so now I set a combination to change activities also. But as I mentioned the switch still influence the other screen making a screen to screen switch and so the bad thing happen again. After a few switches plasma gets freeze again.

Maybe it's not possible to have same activities on both screens is it? Are you able to do that airdrik?

I haven't gotten the same activity on both screens (and generally don't want to).
I have noted that if you use the mouse actions to switch the activity of one screen to the activity that is on the other screen, that the two screens will swap activities (e.g. Screen A is on activity 1, Screen B is on activity 2, and activity 3 is available, switching Screen A to next activity with mouse will result in Screen A -> activity 2, Screen B -> activity 1). On the other hand, if you use keyboard shortcuts to switch and the 'next' activity would be the one that's on the other screen, it will skip that activity and switch to the next one after that (e.g. same start as previous example, switching Screen A to next activity with keyboard will result in Screen A -> activity 3, Screen B stays the same).

As for the plasma freezes, that would be a bug and needs to be reported (bugs.kde.org. you can also check the openSUSE bugtracker if they have reported any similar bugs).